1. Field of the Invention
The present invention relates to an improved method and apparatus for shimming a tubular supermagnetic of the type employed in magnetic resonance imaging equipment.
2. Description of the Prior Art
Magnetic resonance imaging (MRI) equipment is gaining acceptance as a medical diagnostic procedure which is non-invasive and does not employ hazardous forms of radiation such as X-ray. In MRI installations, a patient is positioned in the center of a tubular supermagnet. The individual's unique anatomical characteristics can be recorded and visually presented. It is important that the electromagnetic field of the supermagnet be reasonably uniform and free of discontinuities or irregularities in the electromagnetic field. Such irregularities are known as "impurity gradients". The impurity gradients usually are the result of magnetic metal masses in proximity to the supermagnet, for example, structural columns, beams, girders and panels in the building which contains the MRI unit or which is adjacent to the MRI unit, particularly where the MRI unit is mounted on a mobile trailer. The shimming corrections can be carried out in a site-specific manner by positioning appropriate masses of magnetic material (usually steel) at selected locations on the cylindrical surface of the casing in which the tubular supermagnet is mounted. The cylindrical casing normally is mounted within a housing which provides only limited annular space between the casing and the housing. During the shimming operation it is necessary that a technician crawl into the annular space to secure the appropriate magnetic masses on the cylindrical surface of the casing at the required locations. If the shimming is properly carried out, the supermagnet will be effectively compensated so long as the unit remains at the existing site and so long as the peripheral magnetic metal (beams, columns, girders, panels) causing impurity gradients are not altered.
Because of the initial expense and operating costs of MRI units, there is a need to share the use of the units, e.g., by using one MRI unit at different locations. Mobile MRI units are available to satisfy this need. Mobile units are mounted on trailers together with the accompanying computing equipment, power generators, liquified gas cooling systems, et cetera. Each specific location of the trailer presents a different site with unique impurity gradients requiring unique shimming procedures. The shimming materials applied in a prior location should be removed before the shimming is carried out at the new location. Any individual original shimming activity requires several days of on site intensive activity by highly skilled technicians.
The need to establish and re-establish the unique shimming at each location interferes with the desired mobility of a costly MRI serving a number of operating locations.